A common form of beam scanning antenna is the so-called phased array antenna which comprises an array of antenna elements each with an associated phase shifter for changing the phase of the excitation signal. Varying the phase shift for different elements causes the beam to rotate or scan.
L. Shafai's U.S. Pat. No. 4,947,178, issued Aug. 7, 1990, the entire disclosure of which is incorporated herein by reference, discloses such a beam scanning antenna that generates a high gain beam using azimuthal modes. The antenna arrays for radiating these azimuthal modes comprise stacked microstrip disks or circular slots, that are fed separately from a radio frequency source, through a power divider. Beam scanning is accomplished by introducing appropriate phase shifts between the radiating azimuthal modes, i.e. the microstrip disks or slots. A disadvantage of such an antenna is that all of the separate feed circuits must be modified simultaneously, which requires relatively complex control circuitry, and the antenna is quite costly to make.
Multiple arm spiral antennas have been disclosed which are fed at the inner or outer arm ends. Such spiral antennas may require less complex control circuit and be simpler and less costly to make. As disclosed in U.S. Pat. No. 3,039,099 (H.N. Chait et al) and in U.S. Pat. No. 3,949,407 (K.M. Jagdmann and H.R. Phelan), the entire disclosures of which are incorporated herein by reference, when two opposing arms of a spiral antenna are fed with antiphase currents, currents will flow along the arms until they become in-phase at a place called the active region, where the radius is equal to .lambda./2.pi., where .lambda. is the wavelength. During this condition an efficient radiation takes place, generating a beam along the rotation axis of the spiral. This radiation corresponds to the radiation field of the first azimuthal mode. When the antenna has N arms, feeding them at the inner or outer terminals by a progressive phase difference of 2.pi./N, thereby resulting in a total phase rotation at N-arms of 2.pi., again excites the first azimuthal mode that radiates along the antenna axis. Conversely, feeding the antenna arms with a progressive phase difference of 2.pi.m/N, when m is an integer, thereby resulting in a total phase difference of m2.pi. between the N arms, excites the m.sup.th azimuthal mode. This m.sup.th mode radiates an omni-directional pattern with a null along the antenna axis. Feeding all arms, exciting one mode, gives broadband characteristics useful for direction finding and wideband communication, but none of these modes alone generate a directional beam away from the antenna axis.
In the field of direction finding, 4-arm spiral antennas have been disclosed in which a feed network combines the received signals of all four arms at appropriate phases to extract the power of the first two modes. For the first mode, the phase relationships are 0.degree., 90.degree., 180.degree., 270.degree. and for the second mode they are 0.degree., 180.degree., 360.degree., 540.degree.. The combining network adds and subtracts the signals of these two modes to determine the direction of arrival of the radio frequency wave. Generally, such direction finding antennas have a broadband frequency range which renders them unsuitable for many communications applications where a narrow beam is required, for example to communicate with a satellite.
The present invention seeks to eliminate or at least mitigate the foregoing disadvantages and provide an improved beam scanning antenna which is especially suitable for communications.